Pressure-sensitive adhesives (PSA) have been known and used for a long time in the art, as they provide many desirable characteristics such as ease of application and a wide range of performance characteristics. Some of the common pressure-sensitive adhesives are formulations based upon acrylates, natural rubbers, synthetic rubbers, vinyl acetates, and silicones. Acrylic PSAs are of particular utility in that they are relatively low in cost, adhere well to a variety of different surfaces, and can be formulated to build adhesion to a surface. Many examples of acrylic pressure sensitive adhesives are described in Pressure Sensitive Adhesives Technology, 1st Edition, 1996, Istvan Benedek and Luc J. Heymans, which is incorporated here by reference. An important factor affecting the performance of an acrylic pressure sensitive adhesive is the shear adhesion failure temperature, commonly referred to as SAFT. SAFT defines the upper working temperature of an adhesive and is the upper temperature limit at which the adhesive is able to support a certain amount of weight. However, acrylic PSAs typically have poor high temperature performance. In general, the ease of application of an acrylic pressure-sensitive adhesive relates to the ability of the adhesive to flow or conform to a given substrate with only minimal external force being applied. The greater an adhesive's ability to flow and/or reflow over a surface, the greater its ability to bond to the surface due to higher contact area.
Viscoelastic property characterization of adhesives is an important and most successful rheological tool to study the adhesive's performance. Generally, the viscoelasticity of adhesives is most often determined by dynamic mechanical analysis (DMA). In principle, the adhesion process can be followed easily by using viscoelastic and dynamic mechanical rheological measurements, as the formation of bonds between adhesive and substrate and the flow of an adhesive onto a substrate is reflected in the change of viscoelastic properties. For example, the degree to which the adhesive exhibits more solid-like or liquid-like properties is dependent upon both temperature and time. With dynamic mechanical analysis, a sinusoidal force or stress is applied to an adhesive sample and the resulting sinusoidal deformation or strain is monitored. The sample strain response lags behind the input stress wave with respect to time and this lag is known as the phase angle, δ. The ratio of the dynamic stress to the dynamic strain yields the complex modulus, G*, which can be further broken down to yield the storage (or elastic) modulus, G′, and the loss (or viscosity) modulus, G″. The storage modulus, G′, represents the ability of the adhesive to store energy and it is related to the stiffness of the material. The loss modulus, G″ represents the heat dissipated by the adhesive as a result of the material's given molecular motions and this reflects the flow characteristics of the composition. The ratio of the loss and storage moduli provides another useful quantity called tan δ (tan delta), where tan δ=G″/G′. Tan δ is associated with the viscoelasticity of the adhesive where a low value (less than 1.0) indicates more solid-like viscoelasticity and a high value (greater than 1.0) reflects more liquid-like viscoelasticity. The values of G*, G′ and G″ are dependent upon temperature and frequency. Accordingly, the ability of the adhesive to flow at a given temperature will be reflected by its storage and loss moduli and tan δ values.
What is desired is an adhesive composition having improved resistance to deformation in a shear mode at an elevated temperature, i.e., a high SAFT value, as well as ease of application which is reflected in a tan δ value of greater than 0.4 as measured a room temperature.